1. Field of the Invention
The present invention relates to a switching power supply unit, and more particularly, it relates to a switching power supply unit of a ringing choke converter (hereinafter referred to as an RCC) system.
2. Description of the Related Art
In general, electronic equipment such as an electronic calculator or a communication device needs a stable DC voltage. Thus, in order to supply the stable DC voltage from a commercial AC power supply, a switching power supply unit of the RCC system, in which relatively easy formation can be conducted to obtain high efficiency, has been widely used. The structure of such a switching power supply unit will be illustrated referring to FIG. 8.
In this figure, reference numeral 10 indicates a switching power supply unit, which has an input circuit 2, a DC-DC converter circuit 3, a voltage detection circuit 4 and a control circuit 5.
Among these components, the input circuit 2 has a fuse F connected to an AC power supply AC, a filter circuit LF, and a rectifying diode bridge DB.
Additionally, the DC-DC converter circuit 3 has a smoothing capacitor C1 disposed between the output ends of the diode bridge DB of the input circuit 2, a transformer having a primary winding N1, a secondary winding N2 having the polarity opposite to that of the primary winding N1, and a feedback winding Nb with the same polarity as that of the primary winding N1, an FET Q1 as a main switching element connected in series to an end of the primary winding N1 of the transformer T, a starting resistor R1 connected between the other end of the primary winding N1 and the gate as the control terminal of the FET Q1, a resistor R10 connected between the gate and the source of the FET Q1, a rectifying diode D1 connected in series to an end of the secondary winding N2 of the transformer T, and a smoothing capacitor C4 connected between both ends of the secondary winding N2.
The voltage detection circuit 4 disposed on the output side of the DC-DC converter circuit 3 includes a resistor R5, a light-emitting diode PD on the light-emitting side of a photo coupler PC, a shunt regulator Sr, and resistors R6 and R7. Among these components, the resistor R5, the light-emitting diode PD, and the shunt regulator Sr are mutually connected in series and disposed in parallel to the capacitor C4 of the DC-DC converter circuit 3. In addition, the resistors R6 and R7 are also mutually connected in series and similarly disposed in parallel to the capacitor C4. The node of the resistors R6 and R7 is connected to the shunt regulator Sr.
The control circuit 5 includes a resistor R13 and a capacitor C3 connected in series between one end of the feedback winding Nb and the gate of the FET Q1, a transistor Q2 connected between the gate and the source of the FET Q1, a resistor R2 connected between one end of the feedback winding Nb and the base of the transistor Q2, a resistor R3 and a capacitor C2 connected in parallel between the base and the emitter of the transistor Q2, a resistor R4, a diode D2, and, a photo transistor PT on the light-receiving side of the photo coupler PC, which are mutually connected in series between one end of the feedback winding Nb and the base of the transistor Q2.
Next, a description will be given of the operation of a switching power supply unit 10 having such a structure.
First, on startup, voltage is applied to the gate of the FET Q1 via the resistor R1 to turn on the FET Q1. When the FET Q1 is turned on, a power supply voltage is applied to the primary winding N1 of the transformer T, and voltage is generated in the feedback winding Nb in the same direction as that of the voltage generated in the primary winding N1, whereby the FET Q1 is rapidly turned on by a positive feedback. Under this situation, excitation energy is charged in the primary winding N1.
When the base potential of the transistor Q2 reaches a threshold, the transistor Q2 is turned on, whereas the FET Q1 is turned off. This permits the excitation energy charged in the primary winding N1 of the transformer T during the ON time of the FET Q1 to be discharged as electric energy via the secondary winding N2. The energy is supplied to a load after being rectified by the diode D1 and smoothed by the capacitor C4.
In this way, when the excitation energy charged in the primary winding N1 of the transformer T is all discharged via the secondary winding N2, voltage is again generated in the feedback winding Nb, and the FET Q1 is thereby turned on. When the FET Q1 is turned on, voltage is again applied to the primary winding N1 of the transformer T to charge excitation energy in the primary winding N1.
In the switching power supply unit 10, the above-described oscillating operation is repeated.
Under normal conditions, an output voltage on the load side is divided by the resistors R6 and R7, and the divided detection voltage is compared with a reference voltage of the shunt regulator Sr. After this comparison, the amount of fluctuations in the output voltage is amplified by the shunt regulator Sr, and current flowing through the light-emitting diode PD of the photo coupler PC changes, so that the impedance of the photo transistor PT changes according to the light-emitting amount of the light-emitting diode PD. This operation permits the time for charging/discharging the capacitor C2 to be changed, so that the output voltage is controlled to be fixed.
In the conventional switching power supply unit 10, however, as a characteristic of the RCC system, as shown in FIG. 4(d), the switching frequency of the FET Q1 varies approximately inversely with a load power. Therefore, under light load, the switching frequency is increased, and accordingly, switching losses are increased, whereas under heavy load, the switching frequency is decreased, and accordingly, conduction losses are increased. As a result, these losses lead to reduction in circuit efficiency.